Method and apparatus for updating system information in wireless LAN system

ABSTRACT

The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for updating system information in a wireless LAN system. A method for a station (STA) to update system information in a wireless communication system according to one embodiment of the present invention comprises the steps of: if the value of a change sequence field received by the STA from an access point (AP) is different from the value of the change sequence field stored in the STA, transmitting, to the AP, a probe request frame including the change sequence field set to the value of the change sequence field stored in the STA; and a step of receiving, from the AP, a probe response frame responding to the probe request frame including the change sequence field.

This application is a Continuation of U.S. application Ser. No.14/394,168 filed Oct. 13, 2014, which is a National Stage under 35U.S.C. 371 of International Application No. PCT/KR2013/003135 filed Apr.15, 2013, which claims the benefit of U.S. Provisional Application No.61/623,588 filed Apr. 13, 2012 and 61/676,337 filed Jul. 27, 2012, allof which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for updating systeminformation in a wireless local area network (LAN) system.

BACKGROUND ART

Recently, with development of information communication technology,various wireless communication technologies have been developed. Amongothers, a wireless local area network (WLAN) enables wireless access tothe Internet using a portable terminal such as a personal digitalassistant (PDA), a laptop, a portable multimedia player (PMP) in a home,an enterprise or a specific service provision area based on radiofrequency technology.

In order to overcome limitations in communication rate which have beenpointed out as weakness of a WLAN, in recent technical standards, asystem for increasing network speed and reliability and extendingwireless network distance has been introduced. For example, in IEEE802.11n, multiple input and multiple output (MIMO) technology usingmultiple antennas in a transmitter and a receiver has been introduced inorder to support high throughput (HT) with a maximum data rate of 540Mbps or more, to minimize transmission errors, and to optimize datarate.

DISCLOSURE Technical Problem

As next-generation communication technology, machine-to-machine (M2M)communication technology has been discussed. Even in an IEEE 802.11 WLANsystem, technical standards supporting M2M communication have beendeveloped as IEEE 802.11ah. In M2M communication, a scenario in which asmall amount of data is communicated at a low rate may be considered inan environment in which many apparatuses are present.

Communication in a WLAN system is performed in a medium shared betweenall apparatuses. As in M2M communication, when the number of apparatusesis increased, if a long period of time is taken for channel access ofone device, overall system performance may be degraded and power savingof each device may be impeded.

An object of the present invention devised to solve the problem lies ina new mechanism for updating system information.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Technical Solution

The object of the present invention can be achieved by providing amethod for updating system information by a station (STA) of a wirelesscommunication system, the method including transmitting a probe requestframe including a change sequence field set as a value of a changesequence field stored in the STA, to the AP when a value of a changesequence field received from an access point (AP) by the STA isdifferent from the value of the change sequence field stored in the STA,and receiving a probe response frame, including the change sequencefield, responding to the probe request frame from the AP.

In another aspect of the present invention, provided herein is a methodfor providing system information updated in an access point (AP) of awireless communication system, the method including receiving a proberequest frame including a change sequence field from a station (STA),and transmitting a probe response frame, including the change sequencefield, responding to the probe request frame to the STA, wherein theprobe request frame is received from the STA by the AP when a value of achange sequence field received from the AP by the STA is different froma value of a change sequence field stored in the STA, and the value ofthe change sequence field included in the probe request frame is set asthe value of the change sequence field stored in the STA.

In another aspect of the present invention, provided herein is a station(STA) apparatus for updating system information in a wirelesscommunication system, the STA apparatus including a transceiver, and aprocessor, wherein the processor is configured to transmit a proberequest frame including a change sequence field set as a value of achange sequence field stored in the STA, to the AP using the transceiverwhen a value of a change sequence field received from an access point(AP) by the STA is different from the value of the change sequence fieldstored in the STA and to receive a probe response frame, including thechange sequence field, responding to the probe request frame from the APusing the transceiver.

In another aspect of the present invention, provided herein is an accesspoint (AP) apparatus for providing system information updated in awireless communication system, the AP apparatus including a transceiver,and a processor, wherein the processor is configured to receive a proberequest frame including a change sequence field from a station (STA)using the transceiver and to transmit a probe response frame, includingthe change sequence field, responding to the probe request frame to theSTA using the transceiver, the probe request frame is received from theSTA by the AP when a value of a change sequence field received from theAP by the STA is different from a value of a change sequence fieldstored in the STA, and the value of the change sequence field includedin the probe request frame is set as the value of the change sequencefield stored in the STA.

The embodiments of the present invention may have the followingfeatures.

The change sequence field received from the AP by the STA may beincluded in a short beacon frame transmitted from the AP.

The change sequence field stored in the STA may be a change sequencefield received before the short beacon frame is received from the AP bythe STA.

The probe request frame including the change sequence field may be aprobe request frame for acquisition of updated system information.

The probe response frame may include one or more elements of systeminformation to be updated by the STA when a current change sequencevalue of the AP is different from a value of a change sequence includedin the probe request frame.

The probe response frame may include only the one or more elements ofthe system information to be updated by the STA and the change sequencefield set as a value corresponding to current system information.

The probe request frame including the change sequence field may not betransmitted when a value of a change sequence field received by the STAfrom the AP is the same as the value of the change sequence field storedin the STA.

The probe response frame including one or more elements of systeminformation to be updated by the STA may not be received when a value ofa current change sequence of the AP is the same as a value of a changesequence included in the probe request frame.

The change sequence field may be a field indicating change in the systeminformation.

The change sequence field may be defined as octet size 1 and configuredas one value of 0 to 255.

The value of the change sequence field may be increased by 1 when atleast one element of the system information is changed.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

The present invention may provide a new method and apparatus forupdating system information.

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description taken in conjunction with theaccompanying drawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable;

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable;

FIG. 3 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable;

FIG. 4 is a diagram showing an exemplary structure of a wireless localarea network (WLAN) system;

FIG. 5 is a diagram illustrating a link setup process in a WLAN system;

FIG. 6 is a diagram illustrating a backoff process;

FIG. 7 is a diagram illustrating a hidden node and an exposed node;

FIG. 8 is a diagram illustrating request to send (RTS) and clear to send(CTS);

FIG. 9 is a diagram illustrating power management operation;

FIGS. 10 to 12 are diagram illustrating operation of a station (STA)which receives a TIM;

FIG. 13 is a diagram illustrating a group based association ID (AID);

FIG. 14 is a diagram for explanation of a short beacon;

FIG. 15 is a diagram for explanation of exemplary fields included in ashort beacon frame;

FIG. 16 is a diagram illustrating the format of a short beacon frameaccording to an example of the present invention;

FIG. 17 is a diagram illustrating the format of a short beacon frameaccording to another example of the present invention;

FIG. 18 is a diagram for explanation of a method for transmitting andreceiving a full beacon frame according to an example of the presentinvention;

FIG. 19 is a diagram for explanation of a method for transmitting andreceiving a full beacon frame according to another example of thepresent invention;

FIG. 20 is a diagram for explanation of a method for transmitting andreceiving a full beacon frame according to another example of thepresent invention;

FIG. 21 is a diagram for explanation of transmission of a probe responseframe in a broadcast manner;

FIG. 22 is a diagram showing a change sequence field;

FIG. 23 is a diagram for explanation of a probe request/responseprocedure according to an example of the present invention;

FIG. 24 is a diagram for explanation of a probe request/responseprocedure according to another example of the present invention;

FIG. 25 is a diagram for explanation of a probe request/responseprocedure according to another example of the present invention;

FIG. 26 is a diagram for explanation of a system information (SI) updaterequest/response procedure according to an example of the presentinvention;

FIG. 27 is a diagram for explanation of a method for updating systeminformation using a full beacon request frame; and

FIG. 28 is a block diagram showing the configuration of a wirelessapparatus according to one embodiment of the present invention.

BEST MODE

The invention now will be described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

Embodiments described hereinbelow are combinations of elements andfeatures of the present invention. The elements or features may beconsidered selective unless otherwise mentioned. Each element or featuremay be practiced without being combined with other elements or features.Further, an embodiment of the present invention may be constructed bycombining parts of the elements and/or features. Operation ordersdescribed in embodiments of the present invention may be rearranged.Some constructions of any one embodiment may be included in anotherembodiment and may be replaced with corresponding constructions ofanother embodiment.

Specific terms used in the embodiments of the present invention areprovided to aid in understanding of the present invention. Thesespecific terms may be replaced with other terms within the scope andspirit of the present invention.

In some cases, to prevent the concept of the present invention frombeing obscured, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. In addition, whereverpossible, the same reference numbers will be used throughout thedrawings and the specification to refer to the same or like parts.

The embodiments of the present invention can be supported by standarddocuments disclosed for at least one of wireless access systems,institute of electrical and electronics engineers (IEEE) 802, 3^(rd)generation partnership project (3GPP), 3GPP long term evolution (3GPPLTE), LTE-advanced (LTE-A), and 3GPP2. Steps or parts that are notdescribed to clarify the technical features of the present invention canbe supported by those documents. Further, all terms as set forth hereincan be explained by the standard documents.

Techniques described herein can be used in various wireless accesssystems such as code division multiple access (CDMA), frequency divisionmultiple access (FDMA), time division multiple access (TDMA), orthogonalfrequency division multiple access (OFDMA), single carrier-frequencydivision multiple access (SC-FDMA), etc. CDMA may be implemented as aradio technology such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be implemented as a radio technology such as GlobalSystem for mobile communications (GSM)/general packet radio service(GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMA may beimplemented as a radio technology such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, evolved-UTRA (E-UTRA) etc. For clarity,this application focuses on the IEEE 802.11 system. However, thetechnical features of the present invention are not limited thereto.

Structure of WLAN System

FIG. 1 illustrates an exemplary structure of an IEEE 802.11 system towhich the present invention is applicable.

IEEE 802.11 can be composed of a plurality of components and provide aWLAN supporting STA mobility transparent for higher layers according tointeraction of the components. A basic service set (BSS) may correspondto a basic component block in an IEEE 802.11 LAN. FIG. 1 shows 2 BSSs(BSS1 and BSS2) each of which includes 2 STAs as members (STA1 and STA2being included in BSS1 and STA3 and STA4 being included in BSS2). InFIG. 1, an oval that defines a BSS indicates a coverage area in whichSTAs belonging to the corresponding BSS perform communication. This areamay be called a basic service area (BSA). When an STA moves out of theBSA, the STA cannot directly communicate with other STAs in the BSA.

A most basic BSS in the IEEE 802.11 LAN is an independent BSS (IBSS).For example, the IBSS can have a minimum configuration including only 2STAs. The IBSS has a simplest form and corresponds to the BSS (BSS1 orBSS2) shown in FIG. 1, in which components other than STA are omitted.This configuration is possible when STAs can directly communicate witheach other. This type of LAN can be configured as necessary rather thanbeing previously designed and configured and may be called an ad-hocnetwork.

When an STA is turned on or off, or enters or exits the coverage of aBSS, membership of the STA in the BSS can be dynamically changed. Tobecome a member of the BSS, the STA can join the BSS using asynchronization process. To access all services based on the BSS, theSTA needs to associate with the BSS. Association may be dynamically setand may use a distribution system service (DSS).

FIG. 2 illustrates another exemplary configuration of an IEEE 802.11system to which the present invention is applicable. FIG. 2 shows adistribution system (DS), a distribution system medium (DSM) and anaccess point (AP) in addition to the configuration of FIG. 1.

In a LAN, a direct station-to-station distance may be limited by PHYperformance. While this distance limit can be sufficient in some cases,communication between stations having a long distance therebetween maybe needed in some cases. The DS may be configured to support an extendedcoverage.

The DS refers to a structure in which BSSs are connected to each other.Specifically, BSSs may be present as components of an extended form of anetwork composed of a plurality of BSSs rather than being independentlypresent as shown in FIG. 1.

The DS is a logical concept and may be specified by characteristics ofthe DSM. IEEE 802.11 logically discriminates a wireless medium (WM) fromthe DSM. The logical media are used for different purposes and used bydifferent components. IEEE 802.11 does not limit the media as the samemedium or different media. The fact that plural media are logicallydifferent from each other can explain flexibility of IEEE 802.11 LAN (DSstructure or other network structures). That is, the IEEE 802.11 LAN canbe implemented in various manners and physical characteristics ofimplementations can independently specify corresponding LAN structures.

The DS can support mobile devices by providing seamless integration of aplurality of BSSs and logical services necessary to handle addresses toa destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and has STA functionality. Data can be transmitted betweena BSS and the DS through the AP. For example, STA2 and STA3 have STAfunctionality and provide a function of enabling associated STAs (STA1and STA4) to access the DS. Furthermore, all APs are addressableentities because they basically correspond to an STA. An address used byan AP for communication on the WM is not necessarily equal to an addressused by the AP for communication on the DSM.

Data transmitted from one of STAs associated with an AP to an STAaddress of the AP can be received at an uncontrolled port at all timesand processed by an IEEE 802.1X port access entity. Furthermore, thetransmitted data (or frame) can be delivered to the DS when a controlledport is authenticated.

FIG. 3 illustrates another exemplary configuration of an IEEE 802.11system to which the present invention is applicable. FIG. 3 shows anextended service set (ESS) for providing an extended coverage inaddition to the configuration of FIG. 2.

A wireless network having an arbitrary size and complexity may becomposed of a DS and an ESS. This type of network is called an ESSnetwork in IEEE 802.11. The ESS may correspond to a set of BSSsconnected to a DS. However, the ESS does not include the DS. The ESSnetwork looks like an IBSS network at a logical link control (LLC)layer. STAs belonging to the ESS can communicate with each other andmobile STAs can move from a BSS to another BSS (in the same ESS)transparently to LCC.

IEEE 802.11 does not define relative physical positions of BSSs in FIG.3 and the BSSs may be located as follows. The BSSs can partiallyoverlap, which is a structure normally used to provide continuouscoverage. The BSSs may not be physically connected to each other andthere is a limit on the logical distance between the BSSs. In addition,the BSSs may be physically located at the same position in order toprovide redundancy. Furthermore, one (or more) IBSS or ESS networks maybe physically located in the same space as one (or more ESS) network.This may correspond to an ESS network form when an ad-hoc networkoperates in the location of the ESS network, IEEE 802.11 networks, whichphysically overlap, are configured by different organizations or two ormore different access and security policies are needed at the sameposition.

FIG. 4 illustrates an exemplary configuration of a WLAN system. FIG. 4shows an example of a BSS based on a structure including a DS.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, STAs are devices operating according to MAC/PHY regulations ofIEEE 802.11. The STAs include an AP STA and a non-AP STA. The non-AP STAcorresponds to a device directly handled by a user, such as a laptopcomputer, a cellular phone, etc. In the example of FIG. 4, STA1, STA3and STA4 correspond to the non-AP STA and STA2 and STA5 correspond tothe AP STA.

In the following description, the non-AP STA may be called a terminal,wireless transmit/receive unit (WTRU), user equipment (UE), mobilestation (MS), motile terminal, mobile subscriber station (MSS), etc. TheAP corresponds to a base station (BS), node-B, evolved node-B, basetransceiver system (BTS), femto BS, etc in other wireless communicationfields.

Link Setup Process

FIG. 5 is a diagram illustrating a general link setup process.

In order to establish a link with respect to a network and perform datatransmission and reception, an STA discovers the network, performsauthentication, establishes association and performs an authenticationprocess for security. The link setup process may be referred to as asession initiation process or a session setup process. In addition,discovery, authentication, association and security setup of the linksetup process may be collectively referred to as an association process.

An exemplary link setup process will be described with reference to FIG.5.

In step S510, the STA may perform a network discovery operation. Thenetwork discovery operation may include a scanning operation of the STA.That is, the STA discovers the network in order to access the network.The STA should identify a compatible network before participating in awireless network and a process of identifying a network present in aspecific area is referred to as scanning.

The scanning method includes an active scanning method and a passivescanning method.

In FIG. 5, a network discovery operation including an active scanningprocess is shown. In active scanning, the STA which performs scanningtransmits a probe request frame while moving between channels and waitsfor a response thereto, in order to detect which AP is present. Aresponder transmits a probe response frame to the STA, which transmittedthe probe request frame, as a response to the probe request frame. Theresponder may be an STA which lastly transmitted a beacon frame in a BSSof a scanned channel. In the BSS, since the AP transmits the beaconframe, the AP is the responder. In the IBSS, since the STAs in the IBSSalternately transmit the beacon frame, the responder is not fixed. Forexample, the STA which transmits the probe request frame on a firstchannel and receives the probe response frame on the first channelstores BSS related information included in the received probe responseframe, moves to a next channel (e.g., a second channel) and performsscanning (probe request/response transmission/reception on the secondchannel) using the same method.

Although not shown in FIG. 5, a scanning operation may be performedusing a passive scanning method. In passive scanning, the STA whichperforms scanning waits for a beacon frame while moving betweenchannels. The beacon frame is a management frame in IEEE 802.11 and isperiodically transmitted in order to indicate presence of a wirelessnetwork and to enable the STA, which performs scanning, to discover andparticipate in the wireless network. In the BSS, the AP is responsiblefor periodically transmitting the beacon frame. In the IBSS, the STAsalternately transmit the beacon frame. The STA which performs scanningreceives the beacon frame, stores information about the BSS included inthe beacon frame, and records beacon frame information of each channelwhile moving to another channel. The STA which receives the beacon framemay store BSS related information included in the received beacon frame,move to a next channel and perform scanning on the next channel usingthe same method.

Active scanning has delay and power consumption less than those ofpassive scanning.

After the STA has discovered the network, an authentication process maybe performed in step S520. Such an authentication process may bereferred to as a first authentication process to be distinguished from asecurity setup operation of step S540.

The authentication process includes a process of, at the STA,transmitting an authentication request frame to the AP and, at the AP,transmitting an authentication response frame to the STA in responsethereto. The authentication frame used for authenticationrequest/response corresponds to a management frame.

The authentication frame may include information about an authenticationalgorithm number, an authentication transaction sequence number, astatus code, a challenge text, a robust security network (RSN), a finitecyclic group, etc. The information may be examples of informationincluded in the authentication request/response frame and may bereplaced with other information. The information may further includeadditional information.

The STA may transmit the authentication request frame to the AP. The APmay determine whether authentication of the STA is allowed, based on theinformation included in the received authentication request frame. TheAP may provide the STA with the authentication result via theauthentication response frame.

After the STA is successfully authenticated, an association process maybe performed in step S530. The association process includes a processof, at the STA, transmitting an association request frame to the AP and,at the AP, transmitting an association response frame to the STA inresponse thereto.

For example, the association request frame may include information aboutvarious capabilities, beacon listen interval, service set identifier(SSID), supported rates, RSN, mobility domain, supported operatingclasses, traffic indication map (TIM) broadcast request, interworkingservice capability, etc.

For example, the association response frame may include informationabout various capabilities, status code, association ID (AID), supportedrates, enhanced distributed channel access (EDCA) parameter set,received channel power indicator (RCPI), received signal to noiseindicator (RSNI), mobility domain, timeout interval (associationcomeback time), overlapping BSS scan parameter, TIM broadcast response,QoS map, etc.

This information is purely exemplary information included in theassociation request/response frame and may be replaced with otherinformation. This information may further include additionalinformation.

After the STA is successfully authenticated, a security setup processmay be performed in step S540. The security setup process of step S540may be referred to as an authentication process through a robustsecurity network association (RSNA) request/response. The authenticationprocess of step S520 may be referred to as the first authenticationprocess and the security setup process of step S540 may be simplyreferred to as an authentication process.

The security setup process of step S540 may include a private key setupprocess through 4-way handshaking of an extensible authenticationprotocol over LAN (EAPOL) frame. In addition, the security setup processmay be performed according to a security method which is not defined inthe IEEE 802.11 standard.

Evolution of WLAN

As a technical standard recently established in order to overcomelimitations in communication speed in a WLAN, IEEE 802.11n has beendevised. IEEE 802.11n aims at increasing network speed and reliabilityand extending wireless network distance. More specifically, IEEE 802.11nis based on multiple input and multiple output (MIMO) technology usingmultiple antennas in a transmitter and a receiver in order to supporthigh throughput (HT) with a maximum data rate of 540 Mbps or more, tominimize transmission errors, and to optimize data rate.

As WLANs have come into widespread use and applications using the samehave been diversified, recently, there is a need for a new WLAN systemsupporting throughput higher than a data rate supported by IEEE 802.11n.A next-generation WLAN system supporting very high throughput (VHT) is anext version (e.g., IEEE 802.11ac) of the IEEE 802.11n WLAN system andis an IEEE 802.11 WLAN system newly proposed in order to support a datarate of 1 Gbps or more at a MAC service access point (SAP).

The next-generation WLAN system supports a multi-user MIMO (MU-MIMO)transmission scheme by which a plurality of STAs simultaneously accessesa channel in order to efficiently use a radio channel. According to theMU-MIMO transmission scheme, the AP may simultaneously transmit packetsto one or more MIMO-paired STAs.

In addition, support of a WLAN system operation in a whitespace is beingdiscussed. For example, introduction of a WLAN system in a TV whitespace(WS) such as a frequency band (e.g., 54 to 698 MHz) in an idle state dueto digitalization of analog TVs is being discussed as the IEEE 802.11afstandard. However, this is only exemplary and the whitespace may beincumbently used by a licensed user. The licensed user means a user whois allowed to use a licensed band and may be referred to as a licenseddevice, a primary user or an incumbent user.

For example, the AP and/or the STA which operate in the WS shouldprovide a protection function to the licensed user. For example, if alicensed user such as a microphone already uses a specific WS channelwhich is a frequency band divided on regulation such that a WS band hasa specific bandwidth, the AP and/or the STA cannot use the frequencyband corresponding to the WS channel in order to protect the licenseduser. In addition, the AP and/or the STA must stop use of the frequencyband if the licensed user uses the frequency band used for transmissionand/or reception of a current frame.

Accordingly, the AP and/or the STA should perform a procedure ofdetermining whether a specific frequency band in a WS band is available,that is, whether a licensed user uses the frequency band. Determiningwhether a licensed user uses a specific frequency band is referred to asspectrum sensing. As a spectrum sensing mechanism, an energy detectionmethod, a signature detection method, etc. may be used. It may bedetermined that the licensed user uses the frequency band if receivedsignal strength is equal to or greater than a predetermined value or ifa DTV preamble is detected.

In addition, as next-generation communication technology,machine-to-machine (M2M) communication technology is being discussed.Even in an IEEE 802.11 WLAN system, a technical standard supporting M2Mcommunication has been developed as IEEE 802.11ah. M2M communicationmeans a communication scheme including one or more machines and may bereferred to as machine type communication (MTC). Here, a machine meansan entity which does not require direct operation or intervention of aperson. For example, a device including a mobile communication module,such as a meter or a vending machine, may include a user equipment suchas a smart phone which is capable of automatically accessing a networkwithout operation/intervention of a user to perform communication. M2Mcommunication includes communication between devices (e.g.,device-to-device (D2D) communication) and communication between a deviceand an application server. Examples of communication between a deviceand a server include communication between a vending machine and aserver, communication between a point of sale (POS) device and a serverand communication between an electric meter, a gas meter or a watermeter and a server. An M2M communication based application may includesecurity, transportation, health care, etc. If the characteristics ofsuch examples are considered, in general, M2M communication shouldsupport transmission and reception of a small amount of data at a lowrate in an environment in which very many apparatuses are present.

More specifically, M2M communication should support a larger number ofSTAs. In a currently defined WLAN system, it is assumed that a maximumof 2007 STAs is associated with one AP. However, in M2M communication,methods supporting the case in which a larger number of STAs (about6000) are associated with one AP are being discussed. In addition, inM2M communication, it is estimated that there are many applicationssupporting/requiring a low transfer rate. In order to appropriatelysupport the low transfer rate, for example, in a WLAN system, the STAmay recognize presence of data to be transmitted thereto based on atraffic indication map (TIM) element and methods of reducing a bitmapsize of the TIM are being discussed. In addition, in M2M communication,it is estimated that there is traffic having a very longtransmission/reception interval. For example, in electricity/gas/waterconsumption, a very small amount of data is required to be exchanged ata long period (e.g., one month). In a WLAN system, although the numberof STAs associated with one AP is increased, methods of efficientlysupporting the case in which the number of STAs, in which a data frameto be received from the AP is present during one beacon period, is verysmall are being discussed.

WLAN technology has rapidly evolved. In addition to the above-describedexamples, technology for direct link setup, improvement of mediastreaming performance, support of fast and/or large-scale initialsession setup, support of extended bandwidth and operating frequency,etc. is being developed.

Medium Access Mechanism

In a WLAN system according to IEEE 802.11, the basic access mechanism ofmedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordination function (DCF) of IEEE 802.11MAC and employs a “listen before talk” access mechanism. According tosuch an access mechanism, the AP and/or the STA may perform clearchannel assessment (CCA) for sensing a radio channel or medium during apredetermined time interval (for example, a DCF inter-frame space(DIFS)) before starting transmission. If it is determined that themedium is in an idle state as the sensed result, frame transmissionstarts via the medium. If it is determined that the medium is in anoccupied state, the AP and/or the STA may set and wait for a delayperiod (e.g., a random backoff period) for medium access withoutstarting transmission and then attempt to perform frame transmission.Since several STAs attempt to perform frame transmission after waitingfor different times by applying the random backoff period, it ispossible to minimize collision.

In addition, the IEEE 802.11 MAC protocol provides a hybrid coordinationfunction (HCF). The HCF is based on the DCF and a point coordinationfunction (PCF). The PCF refers to a periodic polling method for enablingall reception AP and/or STAs to receive data frames using a pollingbased synchronous access method. In addition, the HCF has enhanceddistributed channel access (EDCA) and HCF controlled channel access(HCCA). The EDCA uses a contention access method for providing dataframes to a plurality of users by a provider and the HCCA uses acontention-free channel access method using a polling mechanism. Inaddition, the HCF includes a medium access mechanism for improvingquality of service (QoS) of a WLAN and may transmit QoS data both in acontention period (CP) and a contention free period (CFP).

FIG. 6 is a diagram illustrating a backoff process.

Operation based on a random backoff period will be described withreference to FIG. 6. If a medium is changed from an occupied or busystate to an idle state, several STAs may attempt data (or frame)transmission. At this time, a method of minimizing collision, the STAsmay select respective random backoff counts, wait for slot timescorresponding to the random backoff counts and attempt transmission. Therandom backoff count has a pseudo-random integer and may be set to oneof values of 0 to CW. Here, the CW is a contention window parametervalue. The CW parameter is set to CWmin as an initial value but may beset to twice CWmin if transmission fails (e.g., ACK for the transmissionframe is not received). If the CW parameter value becomes CWmax, datatransmission may be attempted while maintaining the CWmax value untildata transmission is successful. If data transmission is successful, theCW parameter value is reset to CWmin. CW, CWmin and CWmax values arepreferably set to 2^(n)−1 (n=0, 1, 2, . . . ).

If the random backoff process starts, the STA continuously monitors themedium while the backoff slots are counted down according to the setbackoff count value. If the medium is in the occupied state, countdownis stopped and, if the medium is in the idle state, countdown isresumed.

In the example of FIG. 6, if packets to be transmitted to the MAC ofSTA3 arrive, STA3 may confirm that the medium is in the idle stateduring the DIFS and immediately transmit a frame. Meanwhile, theremaining STAs monitor that the medium is in the busy state and wait.During a wait time, data to be transmitted may be generated in STA1,STA2 and STA5. The STAs may wait for the DIFS if the medium is in theidle state and then count down the backoff slots according to therespectively selected random backoff count values. In the example ofFIG. 6, STA2 selects a smallest backoff count value and STA1 selects alargest backoff count value. That is, the residual backoff time of STA5is less than the residual backoff time of STA1 when STA2 completesbackoff count and starts frame transmission. STA1 and STA5 stopcountdown and wait while STA2 occupies the medium. If occupancy of themedium by STA2 ends and the medium enters the idle state again, STA1 andSTA5 wait for the DIFS and then resume countdown. That is, after theresidual backoff slots corresponding to the residual backoff time arecounted down, frame transmission may start. Since the residual backofftime of STA5 is less than of STA1, STA5 starts frame transmission. IfSTA2 occupies the medium, data to be transmitted may be generated in theSTA5. At this time, STA4 may wait for the DIFS if the medium enters theidle state, perform countdown according to a random backoff count valueselected thereby, and start frame transmission. In the example of FIG.6, the residual backoff time of STA5 accidentally matches the randombackoff time of STA4. In this case, collision may occur between STA4 andSTA5. If collision occurs, both STA4 and STA5 do not receive ACK anddata transmission fails. In this case, STA4 and STA5 may double the CWvalue, select the respective random backoff count values and thenperform countdown. STA1 may wait while the medium is busy due totransmission of STA4 and STA5, wait for the DIFS if the medium entersthe idle state, and start frame transmission if the residual backofftime has elapsed.

Sensing Operation of STA

As described above, the CSMA/CA mechanism includes not only physicalcarrier sensing for directly sensing a medium by an AP and/or an STA butalso virtual carrier sensing. Virtual carrier sensing solves a problemwhich may occur in medium access, such as a hidden node problem. Forvirtual carrier sensing, MAC of a WLAN may use a network allocationvector (NAV). The NAV refers to a value of a time until a medium becomesavailable, which is indicated to another AP and/or STA by an AP and/oran STA, which is currently utilizing the medium or has rights to utilizethe medium. Accordingly, the NAV value corresponds to a period of timewhen the medium will be used by the AP and/or the STA for transmittingthe frame, and medium access of the STA which receives the NAV value isprohibited during that period of time. The NAV may be set according tothe value of the “duration” field of a MAC header of a frame.

A robust collision detection mechanism for reducing collision has beenintroduced, which will be described with reference to FIGS. 7 and 8.Although a transmission range may not be equal to an actual carriersensing range, for convenience, assume that the transmission range maybe equal to the actual carrier sensing range.

FIG. 7 is a diagram illustrating a hidden node and an exposed node.

FIG. 7(a) shows a hidden node, and, in this case, an STA A and an STA Bare performing communication and an STA C has information to betransmitted. More specifically, although the STA A transmits informationto the STA B, the STA C may determine that the medium is in the idlestate, when carrier sensing is performed before transmitting data to theSTA B. This is because the STA C may not sense transmission of the STA A(that is, the medium is busy). In this case, since the STA Bsimultaneously receives information of the STA A and the STA C,collision occurs. At this time, the STA A may be the hidden node of theSTA C.

FIG. 7(b) shows an exposed node and, in this case, the STA B transmitsdata to the STA A and the STA C has information to be transmitted to theSTA D. In this case, if the STA C performs carrier sensing, it may bedetermined that the medium is busy due to transmission of the STA B. Ifthe STA C has information to be transmitted to the STA D, since it issensed that the medium is busy, the STA C waits until the medium entersthe idle state. However, since the STA A is actually outside thetransmission range of the STA C, transmission from the STA C andtransmission from the STA B may not collide from the viewpoint of theSTA A. Therefore, the STA C unnecessarily waits until transmission ofthe STA B is stopped. At this time, the STA C may be the exposed node ofthe STA B.

FIG. 8 is a diagram illustrating request to send (RTS) and clear to send(CTS).

In the example of FIG. 7, in order to efficiently use a collisionavoidance mechanism, short signaling packet such as RTS and CTS may beused. RST/CTS between two STAs may be enabled to be overheard byperipheral STAs such that the peripheral STAs confirm informationtransmission between the two STAs. For example, if a transmission STAtransmits an RTS frame to a reception STA, the reception STA transmits aCTS frame to peripheral UEs to inform the peripheral UEs that thereception STA receives data.

FIG. 8(a) shows a method of solving a hidden node problem. Assume thatboth the STA A and the STA C attempt to transmit data to the STA B. Ifthe STA A transmits the RTS to the STA B, the STA B transmits the CTS tothe peripheral STA A and C. As a result, the STA C waits until datatransmission of the STA A and the STA B is finished, thereby avoidingcollision.

FIG. 8(b) shows a method of solving an exposed node problem. The STA Cmay overhear RTS/CTS transmission between the STA A and the STA B anddetermine that collision does not occur even when the STA C transmitsdata to another STA (e.g., the STA D). That is, the STA B transmits theRTS to all peripheral UEs and transmits the CTS only to the STA A havingdata to be actually transmitted. Since the STA C receives the RTS butdoes not receive the CTS of the STA A, it can be confirmed that the STAA is outside carrier sensing of the STA C.

Power Management

As described above, in a WLAN system, channel sensing should beperformed before an STA performs transmission and reception. When thechannel is always sensed, continuous power consumption of the STA iscaused. Power consumption in a reception state is not substantiallydifferent from power consumption in a transmission state andcontinuously maintaining the reception state imposes a burden on an STAwith limited power (that is, operated by a battery). Accordingly, if areception standby state is maintained such that the STA continuouslysenses the channel, power is inefficiently consumed without any specialadvantage in terms of WLAN throughput. In order to solve such a problem,in a WLAN system, a power management (PM) mode of the STA is supported.

The PM mode of the STA is divided into an active mode and a power save(PS) mode. The STA fundamentally operates in an active mode. The STAwhich operates in the active mode is maintained in an awake state. Theawake state refers to a state in which normal operation such as frametransmission and reception or channel scanning is possible. The STAwhich operates in the PS mode operates while switching between a sleepstate or an awake state. The STA which operates in the sleep stateoperates with minimum power and does not perform frame transmission andreception or channel scanning.

Since power consumption is reduced as the sleep state of the STA isincreased, the operation period of the STA is increased. However, sinceframe transmission and reception is impossible in the sleep state, theSTA may not unconditionally operate in the sleep state. If a frame to betransmitted from the STA, which operates in the sleep state, to the APis present, the STA may be switched to the awake state to transmit theframe. If a frame to be transmitted from the AP to the STA is present,the STA in the sleep state may not receive the frame and may not confirmthat the frame to be received is present. Accordingly, the STA needs toperform an operation for switching to the awake state according to aspecific period in order to confirm presence of the frame to betransmitted thereto (to receive the frame if the frame to be transmittedis present).

FIG. 9 is a diagram illustrating power management operation.

Referring to FIG. 9, an AP 210 transmits beacon frames to STAs within aBSS at a predetermined period (S211, S212, S213, S214, S215 and S216).The beacon frame includes a traffic indication map (TIM) informationelement. The TIM information element includes information indicatingthat buffered traffic for STAs associated with the AP 210 is present andthe AP 210 will transmit a frame. The TIM element includes a TIM used toindicate a unicast frame or a delivery traffic indication map (DTIM)used to indicate a multicast or broadcast frame.

The AP 210 may transmit the DTIM once whenever the beacon frame istransmitted three times. An STA1 220 and an STA2 222 operate in the PSmode. The STA1 220 and the STA2 222 may be switched from the sleep stateto the awake state at a predetermined wakeup interval to receive a TIMelement transmitted by the AP 210. Each STA may compute a time to switchto the awake state based on a local clock thereof. In the example ofFIG. 9, assume that the clock of the STA matches the clock of the AP.

For example, the predetermined awake interval may be set such that theSTA1 220 is switched to the awake state every beacon interval to receivea TIM element. Accordingly, the STA1 220 may be switched to the awakestate (S211) when the AP 210 first transmits the beacon frame (S211).The STA1 220 may receive the beacon frame and acquire the TIM element.If the acquired TIM element indicates that a frame to be transmitted tothe STA1 220 is present, the STA1 220 may transmit, to the AP 210, apower save-Poll (PS-Poll) frame for requesting frame transmission fromthe AP 210 (S221 a). The AP 210 may transmit the frame to the STA1 220in correspondence with the PS-Poll frame (S231). The STA1 220 whichcompletes frame reception is switched to the sleep state.

When the AP 210 secondly transmits the beacon frame, since anotherdevice access the medium and thus the medium is busy, the AP 210 may nottransmit the beacon frame at an accurate beacon interval and maytransmit the beacon frame at a delayed time (S212). In this case, theoperation mode of the STA1 220 is switched to the awake state accordingto the beacon interval but the delayed beacon frame is not received.Therefore, the operation mode of the STA1 220 is switched to the sleepstate again (S222).

When the AP 210 thirdly transmits the beacon frame, the beacon frame mayinclude a TIM element set to a DTIM. Since the medium is busy, the AP210 transmits the beacon frame at a delayed time (S213). The STA1 220 isswitched to the awake state according to the beacon interval and mayacquire the DTIM via the beacon frame transmitted by the AP 210. Assumethat the DTIM acquired by the STA1 220 indicates that a frame to betransmitted to the STA1 220 is not present and a frame for another STAis present. In this case, the STA1 220 may confirm that a frametransmitted thereby is not present and may be switched to the sleepstate again. The AP 210 transmits the beacon frame and then transmitsthe frame to the STA (S232).

The AP 210 fourthly transmits the beacon frame (S214). Since the STA1220 cannot acquire information indicating that buffered traffic thereforis present via reception of the TIM element twice, the wakeup intervalfor receiving the TIM element may be controlled. Alternatively, ifsignaling information for controlling the wakeup interval of the STA1220 is included in the beacon frame transmitted by the AP 210, thewakeup interval value of the STA1 220 may be controlled. In the presentexample, the STA1 220 may change switching of the operation state forreceiving the TIM element every beacon interval to switching of theoperation state every three beacon intervals. Accordingly, since theSTA1 220 is maintained in the sleep state when the AP 210 transmits thefourth beacon frame (S214) and transmits the fifth beacon frame (S215),the TIM element cannot be acquired.

When the AP 210 sixthly transmits the beacon frame (S216), the STA1 220may be switched to the awake state to acquire the TIM element includedin the beacon frame (S224). Since the TIM element is a DTIM indicatingthat a broadcast frame is present, the STA1 220 may not transmit thePS-Poll frame to the AP 210 but may receive a broadcast frametransmitted by the AP 210 (S234). The wakeup interval set in the STA2230 may be set to be greater than that of the STA1 220. Accordingly, theSTA2 230 may be switched to the awake state to receive the TIM element(S241), when the AP 210 fifthly transmits the beacon frame (S215). TheSTA2 230 may confirm that a frame to be transmitted thereto is presentvia the TIM element and transmits the PS-Poll frame to the AP 210 (S241a) in order to request frame transmission. The AP 210 may transmit theframe to the STA2 230 in correspondence with the PS-Poll frame (S233).

For PM management shown in FIG. 9, a TIM element includes a TIMindicating whether a frame to be transmitted to an STA is present and aDTIM indicating whether a broadcast/multicast frame is present. The DTIMmay be implemented by setting a field of the TIM element.

FIGS. 10 to 12 are diagrams illustrating operation of a station (STA)which receives a traffic indication map (TIM).

Referring to FIG. 10, an STA may be switched from a sleep state to anawake state in order to receive a beacon frame including a TIM from anAP and interpret the received TIM element to confirm that bufferedtraffic to be transmitted thereto is present. The STA may contend withother STAs for medium access for transmitting a PS-Poll frame and thentransmit the PS-Poll frame in order to request data frame transmissionfrom the AP. The AP which receives the PS-Poll frame transmitted by theSTA may transmit the frame to the STA. The STA may receive the dataframe and transmit an ACK frame to the AP. Thereafter, the STA may beswitched to the sleep state again.

As shown in FIG. 10, the AP may receive the PS-Poll frame from the STAand then operate according to an immediate response method fortransmitting a data frame after a predetermined time (e.g., a shortinter-frame space (SIFS)). If the AP does not prepare a data frame to betransmitted to the STA during the SIFS after receiving the PS-Pollframe, the AP may operate according to a deferred response method, whichwill be described with reference to FIG. 11.

In the example of FIG. 11, operation for switching the STA from thesleep state to the awake state, receiving a TIM from the AP, contendingand transmitting a PS-Poll frame to the AP is equal to that of FIG. 10.If the data frame is not prepared during the SIFS even when the APreceives the PS-Poll frame, the data frame is not transmitted but an ACKframe may be transmitted to the STA. If the data frame is prepared aftertransmitting the ACK frame, the AP may contend and transmit the dataframe to the STA. The STA may transmit the ACK frame indicating that thedata frame has been successfully received to the AP and may be switchedto the sleep state.

FIG. 12 shows an example in which the AP transmits the DTIM. The STAsmay be switched from the sleep state to the awake state in order toreceive the beacon frame including the DTIM element from the AP. The STAmay confirm that a multicast/broadcast frame will be transmitted via thereceived DTIM. The AP may immediately transmit data (that is, amulticast/broadcast frame) without PS-Poll frame transmission andreception after transmitting the beacon frame including the DTIM. TheSTAs may receive data in the awake state after receiving the beaconframe including the DTIM and may be switched to the sleep state againafter completing data reception.

TIM Structure

In the PM mode management method based on the TIM (or DTIM) protocoldescribed with reference to FIGS. 9 to 12, the STAs may confirm whethera data frame to be transmitted thereto is present via STA identificationincluded in the TIM element. The STA identification may be related to anassociation identifier (AID) assigned to the STA upon association withthe AP.

The AID is used as a unique identifier for each STA within one BSS. Forexample, in a current WLAN system, the AID may be one of values of 1 to2007. In a currently defined WLAN system, 14 bits are assigned to theAID in a frame transmitted by the AP and/or the STA. Although up to16383 may be assigned as the AID value, 2008 to 16383 may be reserved.

The TIM element according to an existing definition is not appropriatelyapplied to an M2M application in which a large number (e.g., more than2007) of STAs is associated with one AP. If the existing TIM structureextends without change, the size of the TIM bitmap is too large to besupported in an existing frame format and to be suitable for M2Mcommunication considering an application with a low transfer rate. Inaddition, in M2M communication, it is predicted that the number of STAs,in which a reception data frame is present during one beacon period, isvery small. Accordingly, in M2M communication, since the size of the TIMbitmap is increased but most bits have a value of 0, there is a need fortechnology for efficiently compressing the bitmap.

As an existing bitmap compression technology, a method of omitting 0which continuously appears at a front part of a bitmap and defining anoffset (or a start point) is provided. However, if the number of STAs inwhich a buffered frame is present is small but a difference between theAID values of the STAs is large, compression efficiency is bad. Forexample, if only frames to be transmitted to only two STAs respectivelyhaving AID values of 10 and 2000 are buffered, the length of thecompressed bitmap is 1990 but all bits other than both ends have a valueof 0. If the number of STAs which may be associated with one AP issmall, bitmap compression inefficiency is not problematic but, if thenumber of STAs is increased, bitmap compression inefficiencydeteriorates overall system performance.

As a method of solving this problem, AIDs may be divided into severalgroups to more efficiently perform data transmission. A specific groupID (GID) is assigned to each group. AIDs assigned based on the groupwill be described with reference to FIG. 13.

FIG. 13(a) shows an example of AIDs assigned based on a group. In theexample of FIG. 13(a), several bits of a front part of the AID bitmapmay be used to indicate the GID. For example, four DIDs may be expressedby the first two bits of the AID of the AID bitmap. If the total lengthof the AID bitmap is N bits, the first two bits (B1 and B2) indicate theGID of the AID.

FIG. 13(a) shows another example of AIDs assigned based on a group. Inthe example of FIG. 13(b), the GID may be assigned according to thelocation of the AID. At this time, the AIDs using the same GID may beexpressed by an offset and a length value. For example, if GID 1 isexpressed by an offset A and a length B, this means that AIDs of A toA+B−1 on the bitmap have GID 1. For example, in the example of FIG.13(b), assume that all AIDs of 1 to N4 are divided into four groups. Inthis case, AIDs belonging to GID 1 are 1 to N1 and may be expressed byan offset 1 and a length N1. AIDs belonging to GID2 may be expressed byan offset N1+1 and a length N2−N1+1, AIDs belonging to GID 3 may beexpressed by an offset N2+1 and a length N3−N2+1, and AIDs belonging toGID 4 may be expressed by an offset N3+1 and a length N4−N3+1.

If the AIDs assigned based on the group are introduced, channel accessis allowed at a time interval which is changed according to the GID tosolve lack of TIM elements for a large number of STAs and to efficientlyperform data transmission and reception. For example, only channelaccess of STA(s) corresponding to a specific group may be granted duringa specific time interval and channel access of the remaining STA(s) maybe restricted. A predetermined time interval at which only access ofspecific STA(s) is granted may also be referred to as a restrictedaccess window (RAW).

Channel access according to GID will be described with reference to FIG.13(c). FIG. 13(c) shows a channel access mechanism according to a beaconinterval if the AIDs are divided into three groups. At a first beaconinterval (or a first RAW), channel access of STAs belonging to GID 1 isgranted but channel access of STAs belonging to other GIDs is notgranted. For such implementation, the first beacon includes a TIMelement for AIDs corresponding to GID 1. A second beacon frame includesa TIM element for AIDs corresponding to GID 2 and thus only channelaccess of the STAs corresponding to the AIDs belonging to GID 2 isgranted during the second beacon interval (or the second RAW). A thirdbeacon frame includes a TIM element for AIDs corresponding to GID 3 andthus only channel access of the STAs corresponding to the AIDs belongingto GID 3 is granted during the third beacon interval (or the third RAW).A fourth beacon frame includes a TIM element for AIDs corresponding toGID 1 and thus only channel access of the STAs corresponding to the AIDsbelonging to GID 1 is granted during the fourth beacon interval (or thefourth RAW). Only channel access of the STAs corresponding to a specificgroup indicated by the TIM included in the beacon frame may be grantedeven in fifth and subsequent beacon intervals (or fifth and subsequentRAWs).

Although the order of GIDs allowed according to the beacon interval iscyclic or periodic in FIG. 13(c), the present invention is not limitedthereto. That is, by including only AID(s) belonging to specific GID(s)in the TIM elements, only channel access of STA(s) corresponding to thespecific AID(s) may be granted during a specific time interval (e.g., aspecific RAW) and channel access of the remaining STA(s) may not begranted.

The above-described group based AID assignment method may also bereferred to as a hierarchical structure of a TIM. That is, an entire AIDspace may be divided into a plurality of blocks and only channel accessof STA(s) corresponding to a specific block having a non-zero value(that is, STAs of a specific group) may be granted. A TIM having a largesize is divided into small blocks/groups such that the STA easilymaintains TIM information and easily manages blocks/groups according toclass, QoS or usage of the STA. Although a 2-level layer is shown in theexample of FIG. 13, a TIM of a hierarchical structure having two or morelevels may be constructed. For example, the entire AID space may bedivided into a plurality of page groups, each page group may be dividedinto a plurality of blocks, and each block may be divided into aplurality of sub-blocks. In this case, as an extension of the example ofFIG. 13(a), the first N1 bits of the AID bitmap indicate a paging ID(that is, a PID), the next N2 bits indicate a block ID, the next N3 bitsindicate a sub-block ID, and the remaining bits indicate the STA bitlocation in the sub-block.

In the following examples of the present invention, various methods ofdividing and managing STAs (or AIDs assigned to the STAs) on apredetermined hierarchical group basis are applied and the group basedAID assignment method is not limited to the above examples.

PPDU Frame Format

The format of physical layer convergence protocol (PLCP) packet dataunit (PPDU) frame includes a short training field (STF), a long trainingfield (LTF), a signal (SIG) field and a data field. The simplest (e.g.,non-high throughput (non-HT)) PPDU frame format may include a legacy-STF(L-STF), a legacy-LTF (L-LTF), an SIG field, and a data field only. Inaddition, the format may include additional (or different types of) STF,LTF, and SIG field between an SIG field and a data field according to atype of PPDU frame format (e.g., HT-mixed format PPDU, HT-greenfieldformat PPDU, very high throughput (VHT) PPDU, etc.).

The STF is a signal for signal detection, automatic gain control (AGC),diversity selection, minute time synchronization, etc. and the LTF is asignal for channel estimation, frequency error estimation, etc. Acombination of the STF and the LTF may be referred to as a PCLP preamblewhich may be a signal for synchronization of an OFDM physical layer andchannel estimation.

The SIG field may include a RATE field, a LENGTH field, etc. The RATEfield may information about data modulation and coding rate. The LENGTHfield may include information about a date length. In addition, the SIGfield may include a parity bit, a SIG TAIL bit, etc.

The data field may include a SERVICE field, a PLCP service data unit(PSDU), and a PPDU TAIL bit and include a padding bit as necessary. Apartial bit of the SERVICE field may be used for synchronization of adescrambler of a receiving end. The PSDU may correspond to a MAC PDUdefined in a MAC layer and include data that is generated/used in ahigher layer. The PPDU TAIL bit may be used to return a state of anencoder to 0. The padding bit may be used to match a length of the datafield in a predetermined unit.

The MAC PDU may be defined according to various MAC frame formats and abasic MAC frame includes a MAC header, a frame body, and a frame checksequence (FCS). The MAC frame may include a MAC PDU and may betransmitted/received through a PSDU of a data region of the PPDU frameformat.

Meanwhile, the format of a null-data packet (NDP) frame refers to theformat of a frame that does not include a data packet. That is, the NDPframe refers to frame format that includes only a PLCP header portion(i.e., STF, LTF, and SIC fields) of general PPDU format and does notinclude the remaining portion (i.e., a data field). The NDP frame may bereferred to as short frame format.

Short Beacon

A general beacon frame may be composed of a MAC header, a frame body,and an FCS. The frame body may include the following fields.

A timestamp field may be for synchronization. All STAs that receive abeacon frame may change/update a local clock thereof according to atimestamp value.

A beacon interval field indicates a time interval of beacon transmissionand is represented in units of time units (TUs). A TU may include a unitof microsecond (μs) and, for example, may be defined as 1024 μs. A pointof time for transmission of a beacon by an AP may be represented astarget beacon transmission time (TBTT). That is, the beacon intervalfield corresponds to a time interval from one transmission time of onebeacon frame to next TBTT. An STA that receives a previous beacon maycalculate a point of time for transmission of a next beacon from thebeacon interval field. In general, a beacon interval may be set as 100TU.

A capability information field includes information about capability ofdevice/network. For example, network type such as ad-hoc, aninfrastructure network, or the like may be indicated through thecapability information field. In addition, the capability informationfield may be used to indicate details of whether polling is supported,encryption, etc.

In addition, SSID, supported rates, a frequency hopping (FH) parameterset, a direct sequence spread spectrum (DSSS) parameter set, acontention free (CF) parameter set, an IBSS parameter set, TIM, countryIE, power constraint, QoS capability, high-throughput (HT) capability,etc. However, the field/information included in the beacon frame isexemplary, and the beacon frame stated in the present invention is notlimited to the above examples.

Unlike the aforementioned general beacon frame, a short beacon frame maybe defined. In order to distinguish the short beacon frame from thegeneral conventional beacon, a general conventional beacon may bereferred to as a short full beacon.

FIG. 14 is a diagram for explanation of a short beacon.

A short beacon interval may be represented in units of TUs and a beaconinterval (i.e., a beacon interval of a full beacon) may be defined as aninteger multiple of the short beacon interval. As illustrated in FIG.14, Full Beacon Interval=N*Short Beacon Interval may be defined (here,N≧1). For example, a short beacon may be transmitted once Or morebetween time when a full beacon is transmitted once and time when a nextfull beacon is transmitted. FIG. 14 illustrates an example in whichShort B is transmitted three times during a full beacon interval.

A STA may determine whether a network being searched by the STA isavailable, using an SSID (or a compressed SSID) included in a shortbeacon. The STA may transmit association request to a MAC address of anAP included in the short beacon transmitted by a network desired by theSTA. In general, a short beacon is more frequently transmitted than afull beacon and thus a non-associated STA can be rapidly associated bysupporting the short beacon. When an STA requires additional informationfor being associated, the STA may transmit a probe request to a desiredAP. In addition, the STA may perform synchronization using timestampincluded in the short beacon. Moreover, the STA may signal whethersystem information (or network information, and hereinafter,system/network information is collectively referred to as “systeminformation”) is changed through the short beacon. When the systeminformation is changed, the STA may acquire the changed systeminformation through a full beacon. In addition, the short beacon mayinclude TIM. That is, the TIM may be provided through the full beacon orprovided through the short beacon.

FIG. 15 is a diagram for explanation of exemplary fields included in ashort beacon frame.

A frame control (FC) field may include protocol version, type, subtype,next full beacon present, SSID present, BSS bandwidth (BSS BW), andsecurity fields. The FC may have octet length 2.

Among subfields of the FC field, a protocol version field may be definedas a bit length 2 and may be basically configured as 0. The type fieldand the subtype field may be defined as bit lengths 2 and 4,respectively, and the type field and the subtype field may indicate afunction of a corresponding frame together (e.g., which may indicatethat a corresponding frame is a short beacon frame). The next fullbeacon present may be defined as bit length 1 and configured as a valueindicating whether a short beacon frame includes a ‘duration to nextfull beacon’ field (or information about next TBTT). An ‘SSID present’field may be defined as bit length 1 and configured as a valueindicating whether a short beacon frame includes a ‘compressed SSID’field. The BSS BW field may be defined as bit length 3 and configured asa value indicating a current operation bandwidth of BSS (e.g., 1, 2, 4,8, or 16 MHz). The security field may be defined as bit length 1 andconfigured as a value indicating whether an AP is an RSNA AP. Theremaining bits (e.g., 2 bits) may be reserved.

Then a source address (SA) field of the short beacon frame may be a MACaddress of an AP that transmits a short beacon. The SA may have octetlength 6.

The timestamp field may include least significant bit (LSB) 4 byte(i.e., 4 octets) of a timestamp of the AP. This is because, even if onlyLSB 4 byte instead of an entire timestamp is provided, an STA (e.g., anassociated STA) that has received a value of the entire timestamp issufficient to perform synchronization using the LSB 4 byte value.

A change sequence field may include information indicating whethersystem information is changed. In detail, when critical information(e.g., full beacon information) of a network is changed, a changesequence counter is increased by 1. This field is defined as octetlength 1.

The ‘duration to next full beacon’ field may or may not be included in ashort beacon. This field may signal time length to time for transmissionof a next full beacon from time for transmission of a correspondingshort beacon to a STA. Thus the STA that listens to the short beacon mayoperate to a next full beacon in a dos (or sleep) mode, t herebyreducing power consumption. In addition, the ‘duration to next fullbeacon’ field may include information indicating next TBTT. A length ofthis field may be defined as, for example, 3 octets.

The ‘compressed SSID’ field may or may not be included in a shortbeacon. This field may include a portion of an SSID of a network or ahashing value of the SSID. An STA that already knows the correspondingnetwork may be permitted to discover the corresponding network using theSSID. A length of this field may be defined as, for example, 4 octets.

The short beacon frame may include an additional or optional field orinformation elements (IEs) in addition to the above exemplary fields.

A forward error correction (FEC) field may be used to examine whethererror occurs in the short beacon frame and configured as an FCS field.This field may be defined as octet length 4.

Improved System Information Updating Method

In a conventional wireless local area network (LAN) environment, an APoperates via a method for periodically transmitting a full beacon frameincluding system information. However, in an enhanced wireless LANenvironment, an AP may operate via a method in which a full beacon frameincluding system information is not periodically transmitted. Forexample, in an environment such as a home LAN, etc., when an associatedSTA is not present, an AP may operate via a method in which a beacon isnot transmitted. In addition, even if a full beacon frame isperiodically transmitted, a ‘duration to next full beacon’ field may notbe included in a short beacon in order to reduce overhead of a shortbeacon. In this case, the AP may configure a value of a ‘next fullbeacon present’ field in an FC field of a short beacon frame as 0 andtransmit a short beacon that does not include the ‘duration to next fullbeacon’ field.

In this case, when the AP does not notify an STA that a full beacon isnot transmitted, the STA may continuously and repeatedly attempt to andfail to receive a full beacon, thereby increasing power consumption ofthe STA. In addition, when information about a point of time when a nextfull beacon can be received is not included in a short beacon, even ifthe STA receives the short beacon, the STA may continuously attempt toreceive a full beacon until the full beacon is actually transmitted,thereby increasing power consumption. Accordingly, when the AP quicklynotifies the STA that the AP does not transmit the full beacon or that anext full beacon is not periodically transmitted, power consumption ofthe STA can be reduced.

In addition, when the STA determines that the AP does not transmit afull beacon, the corresponding STA may obtain system information via aprobe request/response operation without waiting for a full beacon andmay effectively perform an operation for association with thecorresponding AP. For example, when an AP that receives a probe requestframe from an STA may transmit a probe response frame including systeminformation (e.g., an SSID, a supported rate, an FH parameter set, aDSSS parameter set, a CF parameter set, an IBSS parameter set, a countryIE, etc.) to the corresponding STA in response to the probe requestframe. Thus the STA can be associated with the corresponding AP byobtaining the system information provided through the probe responseframe and performing association request/response.

In a conventional wireless LAN operation, since a full beacon includingsystem information is periodically transmitted, when the systeminformation is changed, an STA may obtain the changed system informationby receiving a next beacon. However, in an environment in which a fullbeacon including system information is not periodically transmitted, anSTA may not accurately obtain the changed system information at anappropriate point of time. In this case, the STA cannot accuratelyoperate in a corresponding wireless LAN network.

The present invention proposes a method for accurately obtaining changedinformation and maintaining updated system information by an STA in asystem in which an AP does not periodically transmit a full beacon frame(i.e., a frame including system information).

Embodiment 1

Embodiment 1 relates to a method for notifying an STA of whether an APperiodically transmits a full beacon frame including system information.

For example, information indicating whether a full beacon may beperiodically transmitted is included in a short beacon frame and may besignaled to the STA.

FIG. 16 is a diagram illustrating the format of a short beacon frameaccording to an example of the present invention.

As illustrated in FIG. 16, a ‘full beacon present’ subfield in an FCfield of a short beacon frame may be defined. A ‘full beacon present’field may be configured as a value indicating whether a periodicallytransmitted full beacon is present. For example, when an AP transmits afull beacon (or when the AP periodically transmits a full beacon), avalue of the ‘full beacon present’ field may be configured as 1. When avalue of the ‘full beacon present’ field may be configured as 0, thismeans that the AP does not transmit a full beacon (or the AP does notperiodically transmit a full beacon). When a value of the ‘full beaconpresent’ field is configured as 0, the ‘full beacon present’ subfield inthe FC field of a short beacon may also be configured as a value (e.g.,0) indicating that a ‘duration to next full beacon’ field is not presentin the short field.

FIG. 17 is a diagram illustrating the format of a short beacon frameaccording to another example of the present invention.

As illustrated in FIG. 17, when a value of a ‘next full beacon present’in an FC field of a short beacon is configured as 1 and simultaneously a‘duration to next full beacon’ field has a predetermined value (e.g.,when all bits are configured as 0 or all bits are configured as 1), thismay indicate that a full beacon is not transmitted (or the full beaconis not periodically transmitted). Unlike in the example of FIG. 16 inwhich an explicit field indicating whether a full beacon is present isadditionally defined, the example of FIG. 17 may correspond to a methodof implicitly indicating non-presence of a full beacon when values oflegacy fields constitute a specific combination.

In the example of FIG. 17, when a value of the ‘duration to next fullbeacon’ is configured as 0, an AP does not transmit a full beacon. Inthis case, even if the AP does not transmit a full beacon, the ‘durationto next full beacon’ field needs always to be included in a short beaconframe.

Embodiment 2

Embodiment 2 relates to operations of an AP and an STA according towhether a full beacon is transmitted.

FIG. 18 is a diagram for explanation of a method for transmitting andreceiving a full beacon frame according to an example of the presentinvention.

An AP may not transmit a full beacon when an STA that is associated withthe AP is not present. In this case, the AP may notify an STA that thefull beacon is not transmitted through a specific field of a shortbeacon (e.g., by configuring a value of a ‘duration to next full beacon’field as 0 as in FIG. 17).

Then when the STA that is associated with the AP is present, thecorresponding AP begins to also transmit a full beacon. In this case, avalue of the ‘duration to next full beacon’ field of a short beaconframe may be configured as a value (e.g., a value that is not 0)indicating a point of time for transmission of a next full beacon, andan STA that receives the short beacon may determine a point of time forreception of a next full beacon.

As in the example of FIG. 16, when the AP does not transmit a fullbeacon, the ‘duration to next full beacon’ field is not included in ashort beacon frame, and a value of a ‘next full beacon present’ fieldmay be configured as 0. An STA that receives the short beacon frame maydetermine that a full beacon is not transmitted and thus perform activescanning without being on standby to receive a full beacon. In addition,the STA that determines that a full beacon is not transmitted frominformation contained in the short beacon may be on standby for apredetermined period of time (e.g., 100 ms (i.e., a default beaconinterval)) from a point of time for reception of the short beacon andthen may perform active scanning when the STA does not receive a fullbeacon for the predetermined period of time.

The STA may transmit a probe request frame to the AP via active scanningand receive a probe response frame from the AP in response to the proberequest frame to obtain system information included in the proberesponse frame. In addition, the probe response frame may includeinformation indicating whether system information is changed (e.g.,change sequence (or version) information).

FIG. 19 is a diagram for explanation of a method for transmitting andreceiving a full beacon frame according to another example of thepresent invention.

When an STA determines that an AP does not transmit a full beacon framefrom information contained in a short beacon frame (e.g., as in theexample of FIG. 16 or 17), the STA may request the AP to transmit thefull beacon frame.

To this end, the STA may transmit a full beacon request frame to the AP.The AP that receives the full beacon request frame may begin to transmita full beacon frame in response to the full beacon request frame.

For example, the AP may receive the full beacon request frame from theSTA and then may periodically transmit the full beacon frame for apredetermined period of time or by as much as a predetermined number oftimes. The predetermined period of time/number of times may beconfigured according to a value requested by the STA or configured basedon a pre-configured value according to system characteristics.

FIG. 20 is a diagram for explanation of a method for transmitting andreceiving a full beacon frame according to another example of thepresent invention.

As described with reference to FIG. 18, when the AP that receives thefull beacon request frame from the STA cannot immediately begin totransmit a full beacon frame, the AP may transmit a full beacon responseframe to the STA. The full beacon response frame may include informationfor determination of a point of time for transmission of a next fullbeacon by the STA (e.g., a ‘duration to next full beacon’ field or anext TBTT field). Thus the STA may determine a point of time forreception of a next full beacon.

Although FIGS. 19 and 20 illustrate the example in which the STAtransmits a full beacon request frame to the AP in order to request theAP for a full beacon, this operation may be performed through aconventional probe request frame. That is, the STA that determines thatthe AP does not transmit a full beacon may transmit a probe requestframe to the AP to request the AP to transmit a full beacon. To thisend, the probe request frame may include information indicating thatthat STA requests transmission of a full beacon frame. The AP thatreceives the probe request frame including this information may transmita full beacon frame to the STA. Alternatively, when the AP cannotimmediately transmit the full beacon frame, the AP may transmit a proberesponse frame to provide information for determination of a point oftime for reception of a next full beacon by the STA to the correspondingSTA.

In short, the STA that determines that the AP does not transmit a fullbeacon frame may transmit a full beacon request frame/probe requestframe to the AP in order to request the AP for a full beacon frame. TheAP may transmit a full beacon frame/full beacon response frame/proberesponse frame in response to the full beacon request frame/proberequest frame.

Here, the full beacon frame/probe response frame transmitted to the STAby the AP may be transmitted to each STA in a unicast or broadcastmanner.

FIG. 21 is a diagram for explanation of transmission of a probe responseframe in a broadcast manner.

In a conventional wireless LAN system, a probe response frame istransmitted in response to a probe request frame and transmitted in aunicast manner for only an STA that transmits a probe request frame.However, as proposed according to the present invention, the proberesponse frame can perform a function for providing information about apoint of time for transmission of a next full beacon like a full beaconresponse frame. Thus, for the function, it may be appropriate tobroadcast a probe response frame.

Information for indicating/requesting transmission of a probe responseframe in a broadcast manner (information indicating broadcast of proberesponse in the example of FIG. 21) may be included in the probe requestframe. In this case, an AP may transmit the probe response frame in abroadcast manner.

For example, a value of a reception address field of the probe responseframe may be configured as a broadcast identifier (e.g., a wildcardvalue). In addition, a robustest modulation and coding scheme (e.g.,quadrature phase shift keying 1/12, 2 repetition) may be applied suchthat all STAs in a BSS receive data of the probe response frametransmitted in a broadcast manner.

Embodiment 3

In a conventional wireless LAN operation, since a full beacon includingsystem information is periodically transmitted, when the systeminformation is changed, an STA may obtain the changed system informationby receiving a next beacon. However, in an environment in which a fullbeacon including system information is not periodically transmitted oronly a short beacon is transmitted without transmission of a fullbeacon, even if system information is changed, the STA cannotimmediately update the system information.

The present embodiment proposes a method for updating changedinformation by an STA when the system information is changed in a systemin which a full beacon is not transmitted.

In a wireless LAN system (e.g., an IEEE 802.11ah system) using a shortbeacon frame, a full beacon frame may be defined to contain informationindicating whether system information is changed.

The information indicating whether the system information is changed maybe defined as a change sequence field or a configuration change sequencefield as shown in FIG. 22. The change sequence field may be configuredas a value indicating whether the system information is changed. Indetail, when other system information except for timestamp informationis changed, the change sequence field may be defined to be increased by1 and may have a value of 0 to 255 (i.e., modulo 256 is applied). When avalue of the change sequence included in the beacon or probe responseframe is maintained as a previous value, the STA may determine that theremaining fields included in the beacon frame or probe response frameare not changed and disregard the remaining fields. However, even if avalue of the change sequence is not changed, the STA may operate toobtain a timestamp value.

In addition, according to the present invention, a probe response framemay be defined to include information indicating whether systeminformation is changed (e.g., a change sequence field). That is, when anAP transmits the probe response frame in response to the probe requestframe transmitted by the STA, the AP may contain a change sequencecorresponding to the system information included in the probe responseframe in the probe response frame and transmit the probe response frame.

Accordingly, upon obtaining system information through a full beaconframe or a probe response frame, the STA may store the systeminformation together with a change sequence value related to theobtained system information. Then upon receiving a short beacon frame ora full beacon frame, the STA may compare the value of the changesequence stored in the STA and a change sequence value included in ashort beacon or a full beacon frame. As the comparison result, when thetwo values are the same, the STA may determine that the systeminformation is not changed. As the comparison result, when the twovalues are different, the STA may perform an operation for updating thechanged system information.

Here, when a full beacon frame is transmitted, the STA may obtain thechanged system information through the corresponding full beacon frame.However, when the full beacon frame is not transmitted, the STA cannotobtain the changed system information through the full beacon frame.Accordingly, when the full beacon frame is not transmitted, thefollowing procedures may be performed in order to update the changedsystem information.

Embodiment 3-1

Embodiment 3-1 relates to a method for updating system information usinga probe request/response procedure.

A conventional probe request/response procedure may be performed foractive scanning during AP discovery of an STA. In addition, the presentinvention proposes usage of a probe request/response procedure in orderto update system information. That is, the conventional proberequest/response procedure is performed for association with acorresponding AP by an STA that is not associated with the AP. On theother hand, according to the present invention, an STA that is alreadyassociated with an AP may transmit probe request and receive proberesponse from the AP in order to update system information.

FIG. 23 is a diagram for explanation of a probe request/responseprocedure according to an example of the present invention.

An STA that is associated with an AP may receive a short beacon and thencheck a value of a change sequence to know that system information ischanged. As in the example of FIG. 23, when the value of the changesequence stored in the STA is 1 but a value of a change sequencecontained in a short beacon is 2, the STA may determine that the systeminformation is changed.

In this case, the STA may transmit a probe request frame to the AP.Here, the STA may further contain, in the probe request frame,indicating that the corresponding probe request frame is a probe requestframe for updating system information.

The AP may transmit a probe response frame to the STA in response to theprobe request frame from the STA. In this case, the AP may containcurrent system information (i.e., updated/changed system information) ina probe response frame and provide the probe response frame to the STA.

In addition, the changed system information needs to be commonly appliedto all STAs in a BSS. Thus even if one STA transmits probe request inorder to update the system information, a probe response frame may notbe transmitted in a unicast manner to the one STA and may be transmit ina broadcast manner in order to update system information of another STAin the BSS.

FIG. 24 is a diagram for explanation of a probe request/responseprocedure according to another example of the present invention.

The aforementioned probe response frame may include all systeminformation. That is, information of a current network may be providedto all STA without consideration of previous system information storedin an STA. This is because, since system information provided through aconventional full beacon is system information for all STAs in a BSSinstead of system information for a specific STA, it is appropriate thatthe probe response frame includes all system information, and since aconventional probe response is provided for first association of the STAwith a network, the conventional probe response is appropriate for anenvironment in which the corresponding STA does not have informationrelated to the corresponding network.

However, as proposed according to the present invention, when an STAthat is already associated with an AP and stores information (i.e.,information that is not changed) of a corresponding network performs anoperation for updating system information, it is more preferable toeffectively provide system information. That is, since it is notnecessary for the STA to redundantly provide the same system informationas system information that is previously owned by the STA and resourcewaste can be caused, there is a need for a method for preventing this.

Accordingly, the present invention proposes provision of only changedportion (i.e., only one or more elements of system information to beupdated by the STA) of current system information compared with systeminformation (i.e., previous system information) stored by the STA. Aprobe response frame including only information about change in systeminformation may be referred to as an optimized probe response frame.

Referring to FIG. 24, when a change sequence value that is pre-stored bythe STA is 1 and a change sequence value included in a short beacon froman AP is 2, the STA may determine that the system information ischanged.

When the STA transmits the probe request frame in order to update thesystem information, the STA may contain a change sequence value storedin the STA in the probe request frame and transmit the probe requestframe. In addition, the STA may further contain, in the probe requestframe, information indicating the probe request frame is a probe requestframe for updating system information.

When the probe request frame received by the AP includes a changesequence value (or when the probe request frame includes informationindicating that the change sequence value and the corresponding proberequest frame are used to update system information), the AP may comparecurrent system information and system information (i.e., systeminformation corresponding to a change sequence value stored by the STA)stored by the STA. As the comparison result, only changed portion ofvarious system information may be selected, contained in a proberesponse frame, and provided to the STA. In the example of FIG. 24, uponreceiving a probe request frame including ‘change sequence=1’, the APmay contain only a current value of changed system informationelement(s) in the probe response frame compared with previous systeminformation in ‘change sequence=2’ and transmit the probe response frameto the STA.

FIG. 24 is a diagram for explanation of a probe request/responseprocedure according to another example of the present invention.

In the example of FIG. 25, a short beacon frame transmitted by an AP maybe transmit to a plurality of STAs STA1, STA2, and STA3 in a broadcastmanner. Here, it is assumed that a change sequence value included in theshort beacon frame is 5. In addition, it is assumed that change sequencevalues corresponding to system information stored in each of STA1, STA2,and STA3 are 1, 2, and 2, respectively.

Accordingly, each of the STAs may determine that system information ischanged and transmit a probe request frame including a change sequencefield configured as a value stored in the corresponding STA to the AP.

In the example of FIG. 25, the AP that receives a probe request framemay transmit a probe response frame including the changed systeminformation (i.e., change sequence=5) in a broadcast manner. The proberesponse frame transmitted in a broadcast manner may include allinformation elements of current system information.

Alternatively, an AP that receives a probe request frame from aplurality of STAs may separately (i.e., in a unicast manner) transmit aprobe response frame to each STA. In this case, system informationincluded in the probe response frame for each STA may include a changedportion compared with system information stored in the correspondingSTA. For example, the probe response frame transmitted to STA1 mayinclude only system information (i.e., a current value of changedinformation element(s) compared with previous system information in oneor more of change sequences=2, 3, 4, and 5) corresponding to ‘changesequence=5’ that is changed compared with system informationcorresponding to change sequence=1. For example, a probe response frametransmitted to STA2 or STA3 may include only system information (i.e., acurrent value of information element(s) that is changed compared withprevious system information in one or more of change sequences=3, 4, and5) corresponding to ‘change sequence=5’ that is changed compared withsystem information corresponding to ‘change sequence=2’.

An AP that receives a probe request frame for updating systeminformation from a plurality of STAs may determine whether a proberesponse frame is transmitted in a broadcast manner or a unicast manner.This may be determined in consideration of the amount of changed systeminformation, the number of STAs that request updating of systeminformation, a network jammed state, etc.

Embodiment 3-2

A similar operation to the method for updating system information usingthe conventional probe request frame/probe response frame described inthe aforementioned Embodiment 3-1 can be performed a newrequest/response frame. The new request/response frame may be referredto as a system information update request frame or a system informationupdate response frame. In addition, the new request/response frame mayalso be referred to as system information (SI) update request frame orSI update response frame. However, a scope of the present invention isnot limited to the term ‘new request/response frame’ and includes arequest/response frame referred to as different terms used foroperations proposed according to the present invention.

FIG. 26 is a diagram for explanation of an SI update request/responseprocedure according to an example of the present invention.

The example of FIG. 26 is the same as the example of FIG. 25 except thatthe probe request frame is replaced with an SI update request frame andthe probe response frame is replaced with an SI update response frame,and thus a repeated description will be omitted.

Embodiment 3-3

FIG. 27 is a diagram for explanation of a method for updating systeminformation using a full beacon request frame.

The example of FIG. 27 is different from the example of FIG. 19 in thatan STA transmits a full beacon request frame in consideration of achange sequence included in a short beacon frame.

That is, in the example of FIG. 27, when a change sequence valueincluded in a short beacon frame is different from a change sequencevalue stored by the STA, the STA may determine that system informationis changed. Thus the STA may transmit a full beacon request frame to theAP. That is, even if the STA determine that the AP does not transmit afull beacon frame, when system information is not changed, a full beaconrequest frame may not be transmitted.

The AP that receives the full beacon request frame may begin to transmita full beacon frame in response to the full beacon request frame. Forexample, the AP may receive a full beacon request frame from the STA andthen may periodically transmit a full beacon frame for a predeterminedperiod of time or by as much as a predetermined number of times. Thepredetermined period of time/number of times may be configured accordingto a value requested by the STA or configured based on a pre-configuredvalue according to system characteristics.

Embodiment 4

As proposed according to the above embodiments, upon receiving a requestframe (e.g., a probe request frame or an SI update request frame)including a change sequence value of an STA from the STA, an AP maytransmit a response frame (e.g., a probe response frame or an SI updateresponse frame) including a current value of information element(s) thatis changed from current system information with reference to the changesequence value of the corresponding STA.

In order to determine a changed portion compared with previous systeminformation (e.g., system information stored by the STA) from currentsystem information and to transmit the changed portion, an AO needs tostore system information corresponding to a previous change sequencevalue. Here, the AP may store only an element ID of a changed IE ratherthan storing information element (IE) of the changed system information.

An element ID of changed IE of system information can be given accordingto Table 1 below.

TABLE 1 Information Element Element ID Inclusion of a Channel SwitchAnnouncement 37 Inclusion of an Extended Channel Switch Announcement 60Modification of the EDCA parameters 12 Inclusion of a Quiet element 40Modification of the DSSS Parameter Set  3 Modification of the CFParameter Set  4 Modification of the FH Parameter Set  8 Modification ofthe HT Operation element 45 . . . . . .

When the element ID of the changed IE is given according to Table 1above, a change sequence stored by an AP and an element ID of changed IEmay be mapped to each other according to change in system information.

For example, it is assumed that EDCA parameter of change sequence 1 ischanged, CF parameter of change sequence 2 is changed, HT operationelement of change sequence 3 is changed, and EDCA parameter of changesequence 4 is changed. In this case, the AP may map and store a changesequence value and an element ID corresponding to changed IE. That is,as shown in Table 2 below, information about change in systeminformation may be stored in the AP.

TABLE 2 Change sequence = 1 Element ID = 12 Change sequence = 2 ElementID = 4 Change sequence = 3 Element ID = 45 Change sequence = 4 ElementID = 12

As shown in Table 2 above, an ID of one IE may be mapped and stored foreach change sequence. Assuming that a size of change sequenceinformation is 1 byte (i.e., information for representing one of 256cases and a size of the mapped element ID information is 1 byte, 2 bytesof total storage space is required in order to represent one element IDmapped to one change sequence.

Assuming that system information is changed according to the aboveexample, a system information updating operation may be performed asfollows.

It is assumed that an STA transmits a request frame (e.g., a proberequest frame or an SI update request frame) including ‘changesequence=2’ and in this case, a change sequence corresponding to currentsystem information of a network is 4. In this case, an AP may determinesystem information that is changed compared with system information(i.e., element ID=45 and 12 in Table 2 above) of change sequence 2. Thusthe AP may contain HT operation element and EDCA parameter thatcorrespond to element ID 45 and 12, respectively, in a response frame(e.g., a probe response frame or an SI update response frame) andtransmit the response frame to the STA.

When an ID of a changed element is mapped to a change sequence value andis continuously stored whenever system information is changed, overheadof a memory of an AP may be increased. For example, assuming that a sizeof change sequence information is 1 byte and a sized of element IDinformation is 1 byte, 512 bytes of total storage space is required inorder to store all element ID information mapped to 256 different changesequence values. However, in general, system information is notfrequently changed and thus information (i.e., a change sequence valueand an element ID value mapped thereto) about change in old systeminformation may not be required. That is, when the AP always maintains512 bytes of storage space in order to store information related tosystem information, unnecessary overhead of a memory of the AP may begenerated.

Accordingly, in order to reduce overhead for storing information relatedto change in system information in the AP, the stored information may berefreshed or limited according to conditions such as time, informationnumber, etc.

For example, the AP may limit the stored information according to timecondition. A unit of a predetermined period (e.g., several minutes,several hours, several days, several months, several years, etc.) may bedetermined, the stored information may be maintained for only acorresponding period of time, and expired information may not bemaintained or may be deleted. For example, when information (i.e., achange sequence and an element ID value mapped thereto) about change insystem information is maintained in a unit of one month, the AP may notmaintain information related to change in system information that hasbeen stored for one months. In this case, a size of a storage spacerequired to store information related to change in system information bythe AP may not be maintained. For example, when system information hasbeen changed once for recent one month, a required storage space has 2bytes. However, when system information is chanced 10 times for recentone month, a required storage space has 20 bytes. However, when storedinformation is limited according to time, if system information isfrequently changed, previous system information may not be lost, therebyimproving the stability of management of system information.

As another example, an AP may limit stored information according to anumber condition of change sequence. The maintained number may beconfigured as, for example, 4, 8, 12, 16 . . . . For example, it isassumed that an AP is configured to maintain only informationcorresponding to 8 recent change sequences and a change sequence valueof current system information is 16. In this case, the AP maintainschange sequence=9, 10, . . . , 16 and element ID information mappedthereto but may not maintain or may delete information (i.e., changesequence=8, 7, 6, 5, . . . and element ID information mapped thereto)related to change in previous system information. In this case, astorage space required to store information related to change in systeminformation by the AP may be maintained to 16 bytes of total size.Accordingly, the efficiency of system information management can beimproved.

The time condition and the number condition can be simultaneouslyapplied to a method for storing information related to systeminformation. For example, information related to change in systeminformation for recent one month may be stored and a maximum storagenumber may be limited to 10 such that system information can be managedusing a flexible storage space of 20 bytes or less.

Details described in the above embodiments of the present invention maybe independently applied or two or more embodiments may besimultaneously applied.

FIG. 28 is a block diagram showing the configuration of a wirelessapparatus according to one embodiment of the present invention.

An AP may include a processor 11, a memory 12 and a transceiver 13. AnSTA 20 may include a processor 21, a memory 22 and a transceiver 23. Thetransceivers 13 and 23 may transmit/receive a radio frequency (RF)signal and implement a physical layer according to an IEEE 802 system,for example. The processors 11 and 21 may be respectively connected tothe transceivers 13 and 21 to implement a physical layer and/or an MAClayer according to the IEEE 802 system. The processors 11 and 21 may beconfigured to perform operation according to the above-described variousembodiments of the present invention. In addition, modules implementingoperations of the AP and the STA according to the above-describedembodiments of the present invention may be stored in the memories 12and 22 and may be executed by the processors 11 and 21, respectively.The memories 12 and 22 may be mounted inside or outside the processors11 and 21 to be connected to the processors 11 and 21 by known means,respectively.

The detailed configuration of the AP and the STA can be implemented suchthat the above-described embodiments of the present invention areindependently applied or two or more embodiments are simultaneouslyapplied and descriptions of redundant parts are omitted for clarity.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof.

In a hardware configuration, the methods according to the embodiments ofthe present invention may be achieved by one or more applicationspecific integrated circuits (ASICs), digital signal processors (DSPs),digital signal processing devices (DSPDs), programmable logic devices(PLDs), field programmable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. For example, software code may be stored in a memory unitand executed by a processor. The memory unit is located at the interioror exterior of the processor and may transmit and receive data to andfrom the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While the above-described embodiments of the present invention focus onIEEE 802.11, they are applicable to various mobile communication systemsin the same manner.

The invention claimed is:
 1. A method for providing system informationupdates in an access point (AP) of a wireless communication system, themethod comprising: transmitting, by the AP, a beacon frame comprising achange sequence field to a station (STA), wherein a value of the changesequence field increases when the AP determines at least one systeminformation element is updated, receiving a probe request frame from theSTA when the value of the change sequence field in the beacon frameincreases, wherein the probe request frame includes a change sequenceelement, and transmitting a probe response frame comprising at least oneof a plurality of system information elements when the change sequenceelement in the probe request frame has a value that is different fromthe value of the change sequence field.
 2. The method according to claim1, wherein the beacon frame corresponds to a short beacon frametransmitted by the AP.
 3. The method according to claim 1, wherein theSTA acquires the value of the change sequence field prior to receivingthe beacon frame from the AP.
 4. The method according to claim 3,wherein the value acquired by the STA is different from the increasedvalue included in the beacon frame.
 5. The method according to claim 1,wherein the probe response frame further comprises the change sequencefield set to an updated value.
 6. The method according to claim 1,wherein the AP does not receive the probe request frame comprising thechange sequence field when the value of the change sequence field doesnot increase.
 7. The method according to claim 1, wherein the value ofthe change sequence field has a size of 1 octet and is set to a value of0 to
 255. 8. An access point (AP) apparatus for providing systeminformation updates in a wireless communication system, the AP apparatuscomprising: a transceiver; and a processor that controls the transceiverto: transmit a beacon frame comprising a change sequence field to astation (STA), wherein a value of the change sequence field increaseswhen the AP determines at least one system information element isupdated, receive a probe request frame from the STA when the value ofthe change sequence field in the beacon frame increases, wherein theprobe request frame includes a change sequence element, and transmit aprobe response frame comprising at least one of a plurality of systeminformation elements when the change sequence element in the proberequest frame has a value that is different from the value of the changesequence field.